Latin name: Prunus cerasifera. 
Variety denomination: RI-1.
In the field of plant genetics, we conduct an extensive and continuing plant-breeding program including the organization and asexual reproduction of orchard trees, and of which plums, peaches, nectarines, apricots, cherries, interspecifics and rootstocks are exemplary. It was against this background of our activities that the present variety of rootstock-interstock was originated and asexually reproduced by us in an experimental orchard located near Modesto, Stanislaus County, Calif.
Among the existing varieties of rootstocks which are known to us, and mentioned herein, are xe2x80x98Nemaguardxe2x80x99 Rootstock (non-patented), xe2x80x98Atlas Rootstockxe2x80x99 (U.S. Plant Pat. No. 8,913), xe2x80x98Viking xe2x80x99 Rootstock (U.S. Plant Pat. No. 8,912), Citation(trademark) Rootstock (U.S. Plant Pat. No. 5,112), xe2x80x98Mazzardxe2x80x99 Rootstock (non-patented), and xe2x80x98Mahalebxe2x80x99 Rootstock (non-patented).
It is well known that sweet cherry trees are compatible with a very limited number of rootstocks for commercial production. For many years, the primary rootstocks available for the production of commercial cherry trees have been xe2x80x98Mahalebxe2x80x99 (non-patented) and xe2x80x98Mazzardxe2x80x99 (non-patented). Both of these varieties are undesirably tall, and have a long juveninity period, delaying production of fruit for five to seven years. Additionally, xe2x80x98Mahalebxe2x80x99 is susceptible to phytophthora root rot and over cropping, which results in small fruit size. xe2x80x98Mazzardxe2x80x99 is a stronger tree with lighter production, but is even taller than xe2x80x98Mahalebxe2x80x99.
Due to the limitations of the rootstocks presently available for commercial cherry trees, we attempted to identify a new interstock that would be compatible with a variety of commercially available rootstocks, as well as with sweet cherry varieties.
The present new Myrobalan tree (Prunus cerasifera) was originated by us in our experimental orchard located near Modesto, Calif. as a selection from seed of open pollinated populations of Myrobalan rootstocks. The exact parentage of the new variety is unknown. We planted and maintained a large group of these open pollinated Myrobalan seedlings growing on their own root system in our experimental orchard. We budded and grafted these seedlings to various rootstocks and various varieties of cherries (Prunus avium), searching for an interstock between the various species. We experienced some success with several combinations, but one seedling, now known as xe2x80x98RI-1xe2x80x99, showed particular promise as an interstock. That seedling was tested by budding and grafting and found to be compatible with cherries and several rootstocks, as was thus selected for asexual propagation and commercialization as an interstock for use with cherry scion wood.
xe2x80x98RI-1xe2x80x99 is distinguishable from other cultivars of the species by its compatibility with cherry trees. Specifically, the xe2x80x98RI-1xe2x80x99 cultivar, when used as an interstock, exhibits significantly less overgrowth at the union of the interstock and scion, as compared with other known Myrobalan cultivars, and exhibits no overgrowth at the union of the rootstock and interstock. The new interstock is graft compatible with a variety of prunus rootstocks, which allows growers to select the most desirable rootstock for a particular type of soil.
Asexual reproduction of the new and distinct variety of Myrobalan tree was by budding to Citation(trademark) Rootstock (U.S. Plant Pat. No. 5,112) as performed by us in our experimental orchard located near Modesto, Calif., and shows that reproductions run true to the original tree and all characteristics of the tree and its fruit are established and transmitted through succeeding asexual propagations.
The new variety of Myrobalan tree (Prunus cerasifera) is of medium size, moderately vigorous, upright in growth and graft compatible with a moderate range of prunus species, which makes it available to be used as an interstock between cherries (Prunus avium) and the interspecific rootstock Citation(trademark) (U.S. Plant Pat. No. 5,112) (Prunus salicinaxc3x97Prunus persica). Budding and grafting tests show compatibility with the following rootstocks and scions: xe2x80x98Nemaguardxe2x80x99 Rootstock (non-patented), Citation(trademark) Rootstock (U.S. Plant Pat. No. 5,112), xe2x80x98Vikingxe2x80x99 Rootstock (U.S. Plant Pat. No. 8,912), xe2x80x98Atlasxe2x80x99 Rootstock (U.S. Plant Pat. No. 8,913), xe2x80x98Craigs Crimsonxe2x80x99 Cherry (U.S. Plant Pat. No. 7,320), xe2x80x98Earlisweetxe2x80x99 Cherry (U.S. Plant Pat. No. 9,783), xe2x80x98Royal Rainierxe2x80x99 Cherry (U.S. Plant Pat. No. 10,790) and the following non-patented cherries: xe2x80x98Bingxe2x80x99, xe2x80x98Vanxe2x80x99, xe2x80x98Rainierxe2x80x99, xe2x80x98Black Tartarianxe2x80x99, xe2x80x98Stellaxe2x80x99, xe2x80x98Lapinsxe2x80x99 and 31 seedling selections of cherries in the advanced testing stage. We expect that similar compatibility will be exhibited with other cherry varieties.
Cherries growing on the new Myrobalan interstock grafted to Citation(trademark) rootstock (xe2x80x9cCitation-Myrobalan-cherry composite treexe2x80x9d) have been observed over a fourteen (14) year period. We have observed a moderate variability in the amount of dwarfing effect within cherry varieties when using the same length of interstock. Decreasing the length of interstock reduces the size of the tree in all cherry varieties, some varieties with a greater degree of dwarfing than other varieties. The interstock should be at least about four inches (approximately 10 cm) in length, and should be no more than about 18 inches (approximately 46 cm) in length. A 6 inch (approximately 15 cm) interstock is preferable. For example, xe2x80x98Earlisweetxe2x80x99 Cherry can be reduced to approximately one-third of normal tree size and xe2x80x98Rainierxe2x80x99 Cherry reduced to approximately one-half normal tree size when budded to a 6-inch segment of the new Myrobalan interstock growing on Citation(trademark) Rootstock. The combination of the new Myrobalan interstock with Citation(trademark) Rootstock has increased precociousness with cherries we have tested to start fruit production in the third year. Crop load is relatively heavy.
The Myrobalan interstock union on the above rootstocks, including Citation(trademark), is relatively smooth, while the bud union on the cherry scion is more prone to be slightly over-grown. The amount of over growth varies within cherry varieties. This same type of overgrowth can be found when growing cherries without an interstock on standard cherry rootstocks such as xe2x80x98Mahalebxe2x80x99. We have had no tree loss with the new Myrobalan interstock from breaking at bud union of either cherry or rootstock.
The Citation-Myrobalan-cherry composite tree has been grown in our test orchards and shows promise as commercial fruit tree. Based on our observations of the Citation-Myrobalan-cherry composite tree, we expect that related composite trees will have similar qualities. Such composite trees could include any of the rootstocks named above, or any prunus or interspecific rootstocks that are compatible with the new Myrobalan interstock. The composite trees could further include any of the sweet cherry varieties named above, or any sweet cherry varieties that are compatible with the new Myrobalan interstock.